I. Field of the Invention
The present invention relates to an apparatus for detecting the presence of dew on a chilled mirror.
II. Description of the Prior Art
As is explained in U.S. Pat. No. 3,623,356 issued to Bisberg and assigned to the assignee of the present application, dew point hygrometers are frequently employed for determining the dew point of a gas sample. Typically, a single mirror is exposed to the gas sample, and the formation of dew thereon is detected by means of a light sensitive detector responsive to the light reflected therefrom. The light sensitive detector develops an error signal which is used to control the cooling current provided to a thermoelectric cooling device associated with the mirror to maintain the mirror at the dew point and, thus, maintain a predetermined thickness of condensation on the mirror surface. A temperature measuring device is employed for measuring the temperature of the mirror, thus giving an indication of the dew point of the gas sample.
In conventional prior art dew point hygrometers, condensation is sensed by the change of light reflected from a mirror surface. As contamination builds up on the mirror surface causing an additional reduction in the light level, the hygrometer responds by reducing the level of condensation until eventually there is no longer any condensation and all of the reduction in light level is due to contamination. At this point, the hygrometer erroneously reads a temperature above the dew point of the gas sample.
Accordingly, it has been necessary to periodically rebalance prior art dew point hygrometers to compensate for changes in the reflective characteristics of the mirror due to the accumulation of contamination. The Bisberg patent discloses one effective manner of automatically compensating for build-up of contamination on the reflective surface of a hygrometer mirror whereby the single mirror is periodically raised in temperature above the dew point, and the amount of reflection measured after evaporation of all dew from the mirror is used to indicate the degree of contamination present on the mirror surface.
Other prior art dew point hygrometers attempt to compensate for contamination build-up by employing a second mirror surface maintained at an ambient temperature with a second light sensitive element positioned to receive reflected light rays from the second mirror surface. The second mirror surface and second light sensitive element are used to provide an indication of the amount of contamination on a first mirror surface which is being cooled to detect dew point in the standard manner.
Athough a substantial advancement over the then existing prior art, the Bisberg dew point hygrometer does require periodic heating of a single mirror which interrupts the operation of the hygrometer. Furthermore, dual mirror hygrometers operate on the assumption that the mirror left at ambient temperature collects contamination at the same rate as the cool mirror. Furthermore, dual mirror hygrometers require separate light sensing circuits for each mirror, and any variation in the operation of either circuit creates an undesirable error in dew point measurement. As a result, prior art dew point hygrometers have only limited capacity to provide continuous dew point measurement automatically over an extended period of time.
It is, therefore, an object of the present invention to provide a mirror arrangement for a dew point hygrometer which allows for continuous compensation for contaminate build-up.
Another object of the present invention is to provide a mirror arrangement for a dew point hygrometer which allows for continuous compensation for contaminate build-up and yet provides for highly accurate dew point measurement.
A further object of the present invention is to provide a mirror arrangement for a dew point hygrometer which allows for compensation for contamination build-up and yet automatically provides for continuous unattended dew point measurement over extended periods of time.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.